frame-relay

The vast majority of service-providers provision traffic parameters such
that Bc=CIR. This means that the service-provider's Tc interval = 1 second.
Remember, Tc=Bc/CIR. (This will not be the Tc interval you see on your
Cisco router, however. More on this later.)

For example, a service provider might tell you your parameters are 384kbps
CIR, 384kb Bc (per Tc), and 384k Be (per Tc). What does this mean?

First, let me explain what the difference is between green, amber and red
frames. "Green Frames" constitute traffic that is less than or equal to Bc.
"Amber Frames" constitute traffic exceeding Bc but less than Bc + Be. "Red
Frames" are traffic exceeding "Bc + Be." In the example above, that means
this:

In this example, you could say that the service-provider is policing at
768kbps.

**It used to be more common for service-provider switches to pass a certain
amount of Red Frames through the network (DE =1, just like Amber Frames).
In the face of congestion, service-providers would drop Red Frames first,
and then Amber frames. At this time, of the 100+ customer networks we
monitor and the thousands of PVCs they have, virtually all of them drop all
red frames at the service-provider's edge (that is, the frame never makes
into the provider's network). We work with five major frame-relay
service-providers, and this seems to be the norm. For some PVCs, however,
they do allow red frames. For the sake of elegance, I don't try to
compensate for the allowance of red frames.

CISCO ROUTER FRAME-RELAY MAP-CLASSES

We're going to assume for this article that you have configured a single PVC
under a subinterface.

Well, given the example above, the first thing you must know is that you can
not put in a CIR and Bc combination that would result in a Tc of 1 second.
Consider the following map-class:

If you were to apply this to a sub-interface and then do a "show frame-relay
traffic-shape" on that sub-interface, you would see an interval of 125ms.
This is because a Cisco router will only allow an interval in the range of
10ms - 125ms. It just so happens, Cisco recommends an interval of 125ms for
data-only PVCs.

Careful now: When you enter cir and mincir values, you are entering them in
*kbps.* When you specify Bc and Be in your map-class, you are specifying Bc
and Be for *one Tc interval.*

So, if 1 interval is 125ms (or 1/8 of a second), and the service-provider's
interval is 1 second, then you must divide the service-providers Bc and Be
by 1/8.

The "byte increment" is 6000 bytes. This means the router can sustain
transmitting 6000 bytes per Tc, or 48000 bits every 125ms. That would mean
your sustained rate is 384000bps, which is your guaranteed rate from your
provider. This is your Bc value at work.

The "byte limit" is 12000 bytes. Here you are combining your Bc and Be
values. This means the router can burst up to 12000 bytes per Tc, or 96000
bits every 125ms. This means what? (Remember, frames exceeding 384kbps, but
remaing below 768kbps are Amber frames (DE =1)). This means that in the
absence of congestion along the PVC path, your router can transmit up to
768kbps!!

With this frame map, your traffic should not exceed 768kbps, so you will
remain within the restrictions placed on you by your service-provider while
maxmizing the bandwidth available to you.

So what happens when there is congestion and the full 768kbps is not
available to you? A frame switch in the PVC path will send a BECN frame to
the busy transmitter on one end of the of the PVC. If your router receives
a BECN, it will back off 1/8 of it's current rate. If the congestion is
persistant your router will continue to do this until it reaches mincir
(configured in the map-class), which is your guaranteed rate from the
service-provider. When the BECNs cease, your router will increase your
transmit rate each interval until it reaches 768kbps per second again, or
until it receives another BECN.

Share This Page

Welcome to Velocity Reviews!

Welcome to the Velocity Reviews, the place to come for the latest tech news and reviews.

Please join our friendly community by clicking the button below - it only takes a few seconds and is totally free. You'll be able to chat with other enthusiasts and get tech help from other members.
Sign up now!